In a NAND flash memory (hereinafter referred to as a NAND memory), information is stored by the electric charge amount stored in the floating gate of a memory cell. The stored information can be read out using a result of applying a voltage (read-out voltage) to the memory cell.
The distribution of the electric charge amount stored in the floating gate of the memory cell may change from the initial state. This may change the optimal read-out voltage (the read-out voltage with a few read-out errors).